The objective of this study is to investigate the role and molecular mechanisms of tissue kallikrein in cardiac protection after myocardial infarction in normal and genetically modified animals. Our recent studies have shown that tissue kallikrein gene or protein delivery protects against organ damage in the heart, kidney and brain through anti- oxidative, anti-apoptotic, anti-inflammatory and angiogenic effects. These results indicate that kallikrein is a pleiotropic agent ideal for combined gene and stem cell therapies for cardiovascular diseases. Therefore, we hypothesize that tissue kallikrein gene delivery or kallikrein modified-mesenchymal stem cell (TK-MSC) implantation provides superior benefits in cardiac repair by inhibiting apoptosis and promoting neovascularization and cardiomyocyte regeneration. We intend to fulfill the following specific aims: 1) determine the effect and signaling mechanisms of tissue kallikrein on angiogenesis, arteriogenesis and ventricular remodeling in rats after myocardial infarction, and on migration, growth and capillary tube formation in endothelial cells; 2) determine whether tissue kallikrein directly activates kinin B2 receptors via proteolysis, independent of kinin formation, to prevent cardiomyocyte apoptosis and inflammation and cardiac dysfunction in kininogen- deficient rats after acute myocardial infarction; 3) determine the viability and effects of TK-MSCs on cardiac function as well as their paracrine effects on cardiomyocyte apoptosis and inflammation in rats after acute myocardial infarction; 4) determine the effects of graft TK-MSCs on cardiac repair and remodeling by promoting neovascularization and cardiac regeneration in rats with post-infarction heart failure. Our long-term goal is to develop a novel therapeutic strategy for myocardial repair and regeneration of damaged myocardium using kallikrein gene and cell-based therapies. This study should generate new and important information to provide the impetus for developing therapeutic regimens in the prevention of heart failure. Project Narrative: Our objective is to investigate the molecular mechanisms of tissue kallikrein in cardiac protection in animal models with acute and chronic myocardial infarction. Our long-term goal is to develop a novel therapeutic strategy for myocardial repair and regeneration of damaged myocardium using kallikrein gene- and cell-based therapies. This study should generate important information to provide the impetus for developing therapeutic regimens in the prevention of cardiac dysfunction and heart failure.